1. Field of the Invention
The present invention relates to an optical fiber communication system that carries multiple optical signals in corresponding channels using wavelength division multiplexing technology. In particular, an improved optical receiver permits the use of lower signal-to-noise ratios in each WDM channel so that more margin is available to improve system performance.
2. Description of Related Art
The capacity of long-haul communication systems, such as “undersea” or “submarine” systems, has been increasing at a substantial rate. For example, some long-haul optically amplified undersea communication systems are capable of transferring information at speeds of 10 gigabits per second (Gbps) or greater. Long-haul communication systems, however, are particularly susceptible to noise and pulse distortion given the relatively long distances over which the signals must travel (e.g., generally 600–12,000 kilometers). Because of these long distances, these systems require periodic amplification along the transmission path. In order to maximize the transmission capacity of an optical fiber network, a single fiber is used to carry multiple optical channels known as wavelength division multiplexing (hereinafter a WDM system). For example, a single optical fiber might carry 32 individual optical signals in separate optical channels at corresponding wavelengths evenly spread in the low loss window of an optical fiber, for example between 1540–1564.8 nanometers (e.g., spread in channels on 0.8 nanometer centers).
In a fiber optic network, the fiber itself has associated nonlinearities. At high optical signal powers, the fiber induces phase shifts on the optical signal due to these fiber nonlinearities. The induced phase shifts in the optical signal correspond to wavelength modulation imposed on the optical signal. When different portions of an optical signal have different wavelengths, these different portions propagate along the transmission fiber at different velocities due to dispersion properties inherent in the fiber media. After propagation for a distance, faster portions may overtake and become superimposed on slower portions causing amplitude distortion.
To counter the induced phase shift effects of high signal powers associated with fiber nonlinearities, an optical phase modulation is sometimes imposed on the optical signal at the transmitter in what is referred to as chirped RZ (CRZ). The inherent band spread of the chirped RZ waveform imposes a limit on how closely adjacent WDM channels may be spaced and subsequently the number of channels within a particular spectral band.
Q-Factor is a measurement of the electrical signal-to-noise ratio at a receive circuit in a communication system that describes the system's bit error rate (BER) performance. Q is inversely related to the BER that occurs when a bitstream propagates through the transmission path. The BER increases at low optical signal-to-noise ratios (SNRs) and decreases at high SNRs. A BER below a specified rate can be achieved by designing the transmission system to provide an SNR greater than a predetermined ratio. The predetermined SNR is based on the maximum specified BER. To achieve a low BER, the SNR must be high, and this may require that the signal power be at a level that induces undesired phase distortions due to fiber nonlinearities.
Electrical signal processing such as error correction and detection techniques are also used in communications systems. Such error correction techniques are often used in wireless transmission systems to improve the BER performance and have found increasing use in optical transmission systems. Forward Error Correction (FEC) is one type of error correction which uses a redundancy code computed and inserted into the data stream at the transmitter end. At the receiver end, the data stream is processed to correct bit errors. While the need to transmit the FEC “overhead” bits along with the data negatively impacts transmission capacity of the physical transmission channel by increasing the transmitted bit rate, the net performance of the transmission system is improved with the use of FEC techniques.